VIENNA, June 23 (Reuters) - Cultivation of coca bushes in Bolivia fell around 9 percent last year to its lowest level since 2002, an annual survey in the world's biggest cocaine producer after Peru and Colombia showed.

The joint survey by the United Nations and Bolivia cited eradication efforts by the government for the decline in the cultivation of the leaf used to make cocaine. The agency forecast a further reduction as part of a strategy by the government to reduce surplus output and fight drug trafficking.

"This decline confirms a downward trend over the last three years, during which period coca cultivation dropped by 26 percent," Antonino De Leo, the U.N. Office on Drugs and Crime (UNODC) representative in Bolivia, said on Monday.

The U.N. said in a statement that Bolivia was on track to cut the area of coca cultivation to 20,000 hectares by 2015.

The area under cultivation in 2013 was around 23,000 hectares, down from 25,300 a year earlier.

Bolivia's President Evo Morales, a former coca farmer, has long defended its legal use as an "ancestral rite" for tea, sweets and medicines. On June 13, Morales presented the United Nations Secretary-General Ban Ki-moon with an unusual treat for his 70th birthday - a cake made with coca leaves.

People in the Andean region traditionally chew on coca leaf as a source of energy or as an antidote to altitude sickness.

The UN Convention on Narcotic Drugs banned coca leaf in 1961 along with cocaine, heroin, opium and morphine. Bolivia withdrew from the convention in 2012 Bolivia it to protest against the criminalization of the chewing of coca leaves.

Subsequently the UN granted Bolivia a special dispensation recognising the traditional practice as legal in Bolivia and Bolivia was re-admitted to the convention. (Reporting by Michael Shields; Editing by James Macharia)